1. Field of the Invention
The present invention relates to a method for the fast charging of a battery, in particular a cadmium-nickel (CdNi) type or nickel-metal-hydride (NiMh) type battery. However, this method can be useful for the fast charging of any type of battery if the charging characteristics of these batteries are similar. An object of the invention is also an improved integrated circuit capable of implementing the method of the invention.
To charge a CdNi or NiMh battery swiftly, a constant current must be injected into the battery. This current has to be all the greater as the charging needs to done swiftly. For example, a three-ampere current enables the charging of a one ampere-hour battery in slightly more than twenty minutes. The difficulty arises in determining when to stop the charging, it being known that a few minutes of overcharging reduces battery life through heating. A few tens of minutes of overcharging may put the battery out of operation or destroy it. The method of the invention makes it possible to stop the charging before the heating takes effect and hence before a state of overcharging occurs, this being achieved without the need for a costly temperature sensor.
2. Description of the Related Art
There are many existing methods to determine the end of the fast charging of a battery when it has reached its rated charging value. These methods are characterized by their reaction time after the end of the charging. This time should be as short as possible. These methods are also characterized by their reliability, repeatability and cost.
The most classic method used is the so-called delta-V or negative delta-V method. This method uses the characteristic curve of charging voltage of a battery, for example of the cadmium-nickel type. This curve is shown in FIG. 1a. When the battery is fully charged, or even slightly overcharged, the voltage falls slowly, after having risen continuously during the charging. This drop in voltage is detected by an electronic control circuit which then decides to stop the charging. It is possible, for example, to detect a twenty-millivolt negative delta V to stop the charging. In this case, the charging is stopped at the point A in FIG. 1a.
This method has two drawbacks. Firstly, when the voltage falls, the battery is slightly overcharged and its temperature has already started to rise. It is therefore already slightly late, for it is the heating that is the prime cause of the ageing of the batteries. This is all the more so as the charging is done speedily. The greater the current, the greater the overcharging and the overheating. Secondly, the batteries obtained by the new technologies, based on nickel-hydride, have a delta-V that is far less accentuated than that of cadmium-nickel batteries. This delta-V is therefore far more difficult to detect. Furthermore, these last-named batteries get heated to a far greater extent at the end of the charging and, hence, any overcharging is far more harmful to them. Notably, their life may be considerably shortened. The - delta V method is therefore inapplicable for nickel-hydride batteries and, furthermore, it shortens the life of cadmium-nickel batteries.
A more recent method consists of measuring the temperature of the battery and of deciding to stop the charging when the rate of increase of the temperature exceeds a certain threshold. Indeed, it is seen in FIG. 1d that the temperature remains constant almost throughout the entire charging operation. It starts rising towards the end with increasing steepness, notably in the last part of the curve. With this method for the measurement of temperature, the charging is stopped at a point B, at a point in time when the temperature rises at a speed that is greater than a certain rate fixed in advance.
One drawback of this method is that it calls for a temperature sensor in each battery as well as one or two contacts in addition to the battery/charger connection. This sometimes entails a prohibitive cost. A second drawback of this method is that the temperature is propagated slowly from the core of the battery to its periphery and then to the sensor. In other words, when the charging is stopped, the battery has already had the time to get excessively heated.
These characteristics are recalled in a description given in the article by Charles H. Small, "Nickel-Hydride Cells Avert Environmental Headaches" in the journal EDN, 10th Dec. 1992, pp. 156 to 161.